[0001] The present invention relates to a method and apparatus for removing impurities from
a feed gas stream by adsorption, particularly by temperature swing adsorption.
[0002] The cryogenic purification of air requires a pre-purification step for the removal
of high-boiling and hazardous materials. Principal high-boiling air components include
water and carbon dioxide. If removal of these impurities from ambient air is not achieved
prior to the air separation system, then water and carbon dioxide will freeze out
in cold sections of the separation apparatus (for example in the heat exchangers and
liquid oxygen sump) causing pressure drop and operational problems. Various hazardous
materials including nitrous oxide, acetylene, and other hydrocarbons also must be
removed. High-boiling hydrocarbons are problematic because they concentrate in the
liquid oxygen section of the separation apparatus, resulting in a potential explosive
hazard. In addition, nitrous oxide can form unstable compounds with the hydrocarbons
and this is another potential hazard.
[0003] Adsorption processes are generally preferred for the removal of these impurities
from feed air to cryogenic air separation plants. These adsorption processes include
thermal swing adsorption (described in US Patents 4,541,851 and 5,137,548) and pressure
swing adsorption (described in US Patent 5,232,474) systems. These systems usually
are designed for total water and carbon dioxide removal from ambient air. Adsorbents
selective for water and carbon dioxide are required for these systems.
[0004] Thermal swing adsorption processes typically use layered adsorbent beds in which
the feed air first contacts a water-selective adsorbent such as alumina or silica
gel. The dry, carbon dioxide containing air then contacts a zeolite adsorbent to remove
carbon dioxide to very low levels. Hydrocarbons and nitrous oxide also are removed
by the appropriate adsorbents, typically in layered configuration. When choosing an
adsorbent system it is important to ensure that the adsorbents will effectively remove
all constituents to be removed from the feed gas.
[0005] The term "mass transfer zone" as used herein refers to the section of an adsorbent
bed in which adsorbent loading of the adsorbate is occurring. Ahead of the leading
edge of the mass transfer zone, the gas concentrations of the adsorbed components
are reduced relative to the feed. At the trailing edge of the mass transfer zone and
behind the trailing edge gas phase composition is substantially equal to that of the
feed mixture and the adsorbent is substantially loaded to capacity with the adsorbed
components from the feed mixture. A small mass transfer zone (corresponding to good
kinetic performance) is beneficial and allows a higher adsorbate loading on the adsorbent
before the leading edge of the mass transfer zone breaks through the effluent end
of the adsorbent bed. This results in more efficient adsorbent bed operation. Consequently,
a smaller bed may be used or the onstream time between regenerations may be increased.
[0006] Small adsorbent particles generally provide shorter mass transfer zones than large
adsorbent particles. The prior art processes disclosed in US Patents 4,964,888 and
5,728,198 and European Patent Publication EP-A-449576 improve mass transfer characteristics
by using a layer of smaller adsorbent particles downstream of a layer of larger adsorbent
particles. However, the use of small adsorbent particles can cause a greater pressure
drop across the adsorbent bed and can result in fluidization, adsorbent attrition,
and carryover of fine adsorbent particles.
[0007] In a first aspect, the invention relates to a method for removing a first and a second
minor component from a gas mixture comprising the first and second minor components
and one or more major components, comprising the steps of:
(a) providing a first adsorbent zone containing a first adsorbent material and a second
adsorbent zone containing a second adsorbent material, wherein the selectivity of
the first adsorbent material for the first minor component relative to the second
minor component is greater than the selectivity of the second adsorbent material for
the first minor component relative to the second minor component, and wherein the
average particle diameter of the first adsorbent material and the average particle
diameter of the second adsorbent material are substantially the same;
(b) passing the gas mixture comprising the first and second minor components and the
one or more major components through the first adsorbent zone and subsequently through
the second adsorbent zone; and
(c) withdrawing from the second adsorbent zone a purified gas containing the one or
more major components and depleted in the first and second minor components.
[0008] The first minor component may be nitrous oxide and the second minor component may
be carbon dioxide. The one or more major components may comprise oxygen and nitrogen.
The one or more major components may comprise air.
[0009] The average particle diameter of the first adsorbent material preferably is between
about 85% and about 115% of the average particle diameter of the second adsorbent
material. The average particle diameter of the first adsorbent material may be between
about 0.5 mm and about 5 mm. The first adsorbent material may comprise CaX zeolite
and the second adsorbent material may comprise 13X zeolite.
[0010] The mixture may further comprise water and an additional adsorbent zone may be provided
prior to the first adsorbent zone, and this additional adsorbent zone preferably contains
adsorbent material which selectively adsorbs water from the gas mixture prior to the
first adsorbent zone.
[0011] The feed gas typically is provided at a temperature between about 0°C and about 50°C.
The method may further comprise terminating steps (b) and (c) and regenerating the
first and second adsorbent materials by passing therethrough a regeneration gas at
a temperature between about 80°C and 400°C.
[0012] In a second aspect, the invention relates to a system for removing a first and a
second minor component from a gas mixture comprising the first and second minor components
and one or more major components, which system comprises:
(a) an adsorber vessel having a first adsorbent zone containing a first adsorbent
material and a second adsorbent zone containing a second adsorbent material, wherein
the selectivity of the first adsorbent material for the first minor component relative
to the second minor component is greater than the selectivity of the second adsorbent
material for the first minor component relative to the second minor component, and
wherein the average particle diameter of the first adsorbent material and the average
particle diameter of the second adsorbent material are substantially the same;
(b) an inlet for passing the gas mixture into the adsorber vessel such that the gas
mixture passes through the first adsorbent zone and subsequently through the second
adsorbent zone; and
(c) an outlet for withdrawing from adsorber vessel a purified gas containing the one
or more major components and depleted in the first and second minor components.
[0013] The first minor component may be nitrous oxide and the second minor component may
be carbon dioxide. The one or more major components may comprise oxygen and nitrogen.
The one or more major components may comprise air.
[0014] The average particle diameter of the first adsorbent material preferably is between
about 85% and about 115% of the average particle diameter of the second adsorbent
material. The average particle diameter of the first adsorbent material may be between
about 0.5 mm and about 5 mm. The first adsorbent material may comprise CaX zeolite
and the second adsorbent material may comprise 13X zeolite.
[0015] The gas mixture may further comprise water and an additional adsorbent zone may be
provided prior to the first adsorbent zone, wherein the additional adsorbent zone
preferably contains adsorbent material which selectively adsorbs water from the gas
mixture prior to the first adsorbent zone.
[0016] In a third aspect, the invention relates to a process for removing one or more constituents
from a feed gas stream to form a purified gas stream, comprising the steps of:
passing the feed gas stream through a bed of particles of a first adsorbent and removing
a portion of one of the constituents from the feed gas stream; and subsequently
passing the feed gas stream through a bed of particles of a second adsorbent and removing
another portion of the one of the constituents from the feed gas stream;
wherein the bed of the second adsorbent has a shorter mass transfer zone for the
one of the constituents than the bed of the first adsorbent under the same conditions,
and the particles of the first and second adsorbents have substantially the same average
diameter.
[0017] Preferably, the particles of the first and second adsorbents each have an average
diameter of 2 to 5 mm.
[0018] Preferably, the one of the constituents is carbon dioxide.
[0019] Preferably, another of the one or more constituents is nitrous oxide.
[0020] Preferably, the feed gas stream comprises one or more gases selected from the group
consisting of air, nitrogen, oxygen, carbon monoxide, helium, methane, hydrogen and
argon.
[0021] Preferably, the feed gas stream is at a pressure of 3 to 20 atmospheres.
[0022] Preferably, the process further comprises the step of regenerating the first and
second adsorbents with a regenerating gas.
[0023] Preferably, the regenerating gas comprises one or more gases selected from the group
consisting of oxygen, nitrogen, methane, hydrogen, argon, helium.
[0024] Preferably, the process further comprises the step of passing the feed gas stream
over a desiccant before passing the feed gas stream over the first adsorbent, wherein
the desiccant is selected from the group consisting of alumina, silica gel, impregnated
aluminas, A zeolites and X zeolites.
[0025] Preferably, the process further comprises the step of cryogenic distillation of the
purified gas stream.
[0026] The third aspect of the invention may be practised using an apparatus for removing
a constituent from a feed gas stream to form a purified gas stream, comprising:
a container having an inlet for the feed gas stream and an outlet for the purified
gas stream;
a first adsorbent region within the container comprising particles of a first adsorbent
for adsorbing the constituent to be moved from the feed gas stream; and
a second adsorbent region downstream of the first adsorbent region within the container
comprising particles of a second adsorbent for adsorbing the constituent to be removed
from the feed gas stream;
wherein the second adsorbent has a shorter mass transfer zone than the first adsorbent
under the same conditions, and the particles of the first and second adsorbents have
substantially the same average diameter.
[0027] In a fourth aspect, the invention relates to a process for removing a first and a
second constitutent from a feed gas stream to form a purified gas stream, comprising
the steps of:
passing the feed gas stream through a bed of particles of a first adsorbent, the first
adsorbent having a selectivity for the first constituent over the second constituent
of 0.45 or greater; and subsequently passing the feed gas stream through a bed of
particles of a second adsorbent;
wherein the bed of the second adsorbent has a shorter mass transfer zone than
the bed of the first adsorbent for the second constituent under the same conditions,
and the particles of the first and second adsorbents have substantially the same average
diameter.
[0028] Preferably, the selectivity of the second adsorbent for the first constituent over
the second constituent is 0.40 or less.
[0029] Preferably, the first adsorbent has a selectivity for the first constituent over
the second constituent of 0.48 or greater.
[0030] Optionally, the first adsorbent has a length of unused bed of 15 inches (38cm) or
longer as determined by the method of Example 1 (column diameter 8 inch (20 cm), filling
depth 3 ft (0.9 m), feed gas is air with 385 ppmv carbon dioxide and 315 ppmv nitrous
oxide at 21 °C/5.7 bar, flow rate 41 lbmoles/ft
2/hr (2 x 10
5 moles/(hr•m
2), initial regeneration in flowing nitrogen at 200 °C).
[0031] The features of any aspect of the invention can be used in combination with the features
of any other aspect of the invention.
[0032] Figure 1 is a schematic flowsheet for an exemplary embodiment of the present invention.
[0033] The present invention in its broadest embodiment relates to a method for removing
a first and a second minor component from a gas mixture comprising the first and second
minor components and one or more major components, which method utilizes one or more
adsorber vessels, each having a first adsorbent zone containing a first adsorbent
material and a second adsorbent zone containing a second adsorbent material. The selectivity
of the first adsorbent material for the first minor component relative to the second
minor component preferably is greater than the selectivity of the second adsorbent
material for the first minor component relative to the second minor component.
[0034] The average particle diameter of the first adsorbent material and the average particle
diameter of the second adsorbent material preferably are substantially the same. The
gas mixture comprising the first and second minor components and the one or more major
components is passed through the first adsorbent zone and subsequently through the
second adsorbent zone, wherein the minor components are selectively adsorbed. A purified
gas containing the one or more major components and depleted in the first and second
minor components is withdrawn from the second adsorbent zone. The invention can be
applied to a wide variety of gaseous mixtures containing minor components which are
undesirable impurities to be removed from the major components in the feed gas.
[0035] A minor component is defined as a component which may be present in a gas mixture
at a concentration up to about 1 vol%, and typically this concentration is expressed
as parts per million by volume (ppmv). Minor components may be present at even lower
concentrations and may be reported in the range of parts per billion (ppbv). A major
component is defined as a component present at higher concentrations, typically many
orders of magnitude higher, than the concentrations of the minor components.
[0036] The invention may be illustrated by the removal of impurities from air feed to a
cryogenic air separation process. In this example, referring to the Figure, air to
be purified is supplied to a main air compressor system 10 at an inlet 12 and is compressed
by a multi-stage compressor with inter- and after- cooling by heat exchange with water
(not shown). Optionally, the compressed air feed is subcooled in cooler 8. The cooled
compressed air is supplied to inlet manifold 14 containing inlet control valves 16
and 18 which are connected to via manifold 14 to a pair of adsorbent vessels 20 and
22. Inlet manifold 14 is bridged downstream of the control valves 16 and 18 by a venting
manifold 24 containing venting valves 26 and 28 which serve to close and open connections
between the upstream end of respective adsorbent vessels 20 and 22 and vent 30 via
silencer 32.
[0037] The air feed contains undesirable impurities including water, carbon dioxide, light
hydrocarbons such as methane, ethane, ethylene, and acetylene, and nitrous oxide.
These components must be removed to eliminate pressure drop and plugging problems
due to frozen deposits of water and carbon dioxide and also to eliminate the possibility
of disastrous energy releases resulting from chemical reactions of hydrocarbons and
oxygen. Each of the two adsorbent vessels 20 and 22 typically contains at least two
types of adsorbents: a pretreatment adsorbent for removing water and at least two
adsorbent materials for removing carbon dioxide, nitrous oxide, and hydrocarbons.
The adsorbent vessels 20 and 22 each contain at least lower adsorbent zones or layers
34 and 34', middle adsorbent zones or layers 35 and 35', and upper adsorbent zones
or layers 36 and 36', respectively.
[0038] Adsorbent layers 34 and 34' each contain a pretreatment adsorbent (for example, silica
gel, activated alumina, or 13X zeolite) to adsorb primarily water. Adsorbent layers
35 and 35' each contain a first adsorbent material (for example, a CaX zeolite) which
adsorbs carbon dioxide and nitrous oxide. Other adsorbents which may be used in layers
35 and 35' include calcium mordenite, BaX zeolite, CaLSX zeolite, and binderless CaLSX
zeolite. Adsorbent layers 36 and 36' each contain a second adsorbent material (for
example, a 13X zeolite) which also adsorbs carbon dioxide and nitrous oxide. The pretreatment
adsorbent which adsorbs primarily water also may adsorb some carbon dioxide; the first
and/or second adsorbent materials preferably also adsorb the hydrocarbons described
above.
[0039] The depth of layers 34 and 34' may be in the range of 0.5 to 10 ft, the depth of
layers 35 and 35' may be in the range of 0.5 to 10 ft, and the depth of layers 36
and 36' may be in the range of 0.5 to 10 ft. Preferably the ratio of the depth of
layers 35 and 35' to the depth of layers 36 and 36' is between about 1:10 and about
10:1
[0040] The first adsorbent material preferably has a selectivity for nitrous oxide relative
to carbon dioxide which is greater than the selectivity of the second adsorbent for
nitrous oxide relative to carbon dioxide. Selectivity is defined here as the ratio
of the Henry's law constant for one adsorbed component on the adsorbent material to
the Henry's law constant for the other adsorbed component on the same adsorbent material
at the same conditions. The Henry's law constant is defined as the initial slope of
the isotherm which describes the amount of the component adsorbed as a function of
gas pressure, at a reference temperature of 30°C. The selectivity of nitrous oxide
to carbon dioxide on CaX zeolite is the ratio of the Henry's law constant for nitrous
oxide adsorbed on CaX to the Henry's law constant for carbon dioxide adsorbed on CaX
at the same temperature, and is 0.49. The selectivity of nitrous oxide to carbon dioxide
on UOP 13X zeolite is 0.39. Selectivity values of nitrous oxide to carbon dioxide
on various adsorbents are shown in Table 1. Selectivity is discussed in US 6106593.
Table 1
Adsorbent |
Selectivity of nitrous oxide
to carbon dioxide |
Alcan AA-300 alumina |
0.08 |
UOP 5A |
0.37 |
Binderless CaX |
1.00 |
Na-mordenite |
0.51 |
Ca-mordenite |
0.30 |
BaX |
0.51 |
[0041] The particles of the adsorbent materials described herein can be in the shape of
beads, extrudates, or can be irregular shapes which result from crushing and sieving.
The average particle size of an adsorbent material in the form of beads or irregular
shapes is defined as the weighted mean of the particle size distribution as determined
by standard methods known in the art. One method is fractionating the adsorbent particles
through a series of standard sieve screens as described in the
Chemical Engineers' Handbook, Fifth Edition, by R. H. Perry and C. H. Chilton, Section 21, Screening. The average
particle diameter of extrudates can be calculated by methods given in the
Chemical Engineers' Handbook, Fifth Edition, by R. H. Perry and C. H. Chilton, Section 5, Beds of Solids.
[0042] The average particle size of the first adsorbent material is substantially the same
as the average particle size of the second adsorbent material. Because it may be difficult
or impractical in large-scale operations to obtain two different adsorbent materials
with exactly the same average particle diameter, it is preferable that the average
particle diameters of the two adsorbents be as close as possible within reasonable
economic considerations. The phrase "the average particle diameter of the first adsorbent
material and the average particle diameter of the second adsorbent material are substantially
the same" as used herein means quantitatively that the average particle diameter of
the first adsorbent material is between about 85% and about 115% of the average particle
diameter of the second adsorbent material. Typically, the average particle diameter
of the first adsorbent material is between about 0.5 mm and about 5 mm.
[0043] The pretreatment adsorbent and the first and second adsorbents may be arranged in
layers as shown in the Figure for axial adsorbent beds. Alternatively, the pretreatment
adsorbent and two adsorbent materials may be layered radially in a radial adsorption
bed. It should be understood that vessels 20 and 22 each can be separated into smaller
vessels arranged in series if desired and that references to "layers" above include
arrangements in which the separate adsorbents are placed in separate vessels arranged
in series.
[0044] The apparatus in the Figure has an outlet 38 connected to the downstream ends of
adsorbent vessels 20 and 22 by outlet manifold 40 containing outlet control valves
42 and 44. Outlet 38 provides feed gas to cryogenic air separation unit 45. Outlet
manifold 40 is bridged by regenerating gas manifold 46 containing regenerating gas
control valves 48 and 50. Upstream from the regenerating gas manifold 46, a line 52
containing a control valve 54 also bridges across the outlet manifold 40.
[0045] A regeneration gas is provided via line 56 and control valve 58 to heater 62, and
hot gas is provided to regenerating gas manifold 46. The operation of the valves may
be controlled by suitable programmable timing and valve opening means as known in
the art (not shown).
[0046] In operation air is compressed in main compressor system 10 and is fed to inlet manifold
14 and passes through one of the two adsorbent vessels 20 and 22. Starting from a
position in which air is passing through open valve 16 to adsorbent vessel 20, and
through open valve 42 to the outlet line 38 to air separation unit 45, valve 18 in
the inlet manifold will just have been closed to terminate the flow of feed air to
vessel 22. Valve 44 also will just have closed. At this stage valves 46, 50, 54, 26
and 28 are all closed. Bed 20 thus is operating in the purification mode while bed
22 is operating in the regeneration mode.
[0047] To commence depressurization of bed 22, valve 28 is opened and once the pressure
in the vessel 22 has fallen to a desired level, valve 28 is kept open while valve
50 is opened to begin flow of regeneration gas. The regeneration gas typically will
be a flow of dry, carbon dioxide-free nitrogen obtained from air separation unit 45,
possibly containing small amounts of argon, oxygen and other gases. Valve 58 is opened
so that the regeneration gas is heated, for example to a temperature of about 200°C,
before passing into vessel 22. The exit purge gas flows from the vessel, through manifold
24, silencer 32, and the vent outlet 30 from which it is discharged to the atmosphere.
[0048] At the end of the predetermined regeneration period, valve 58 may be closed to end
the flow of regenerating gas and valve 54 may be opened to displace nitrogen from
the adsorbent and, after the closing of valve 28, to depressurize vessel 22 with purified
air. Thereafter, valve 54 may be closed and valves 18 and 44 may be opened to put
vessel 22 back on line. Vessel 20 may then be regenerated in a similar manner and
the whole sequence continued in repeating cycles with vessels 20 and 22 proceeding
in alternating modes through the steps of air purification, depressurization, regeneration,
and repressurization.
[0049] It will be appreciated that although the invention has been illustrated above with
reference to an example for the purification of air feed to a cryogenic air separation
plant, many variations and modifications of the invention are possible for use in
this and other embodiments for different types of gas mixtures.
EXAMPLE 1
[0050] Laboratory tests were carried out using an 8 inch (20 cm) diameter adsorber vessel
which was filled to a depth of 3 ft (0.9 m) with CaX zeolite with an average particle
diameter of 2.8 mm. The adsorbent initially was regenerated in flowing nitrogen at
200°C. Air containing 385 ppmv carbon dioxide and 315 ppbv nitrous oxide was introduced
into the adsorber in an upflow mode at 41 lbmoles/(hr•ft
2) (2 x 10
5 moles/ (hr•m
2)), a temperature of 21°C, and a pressure of 5.7 bara. The outlet concentration of
carbon dioxide was measured as a function of time, in particular the time between
observed outlet concentrations of 1 ppmv and 385 ppmv (the inlet concentration). The
outlet concentration of nitrous oxide also was measured. The length of unused bed
(LUB) for carbon dioxide was determined from these measurements by known methods such
as those described in Principles of Adsorption and Adsorption Processes by D. Ruthven,
John Wiley and Sons (1984). The length of unused bed is defined as half the length
of the mass transfer zone earlier defined.
[0051] The same experiment was repeated using 13X zeolite with a bed depth of 5 ft (1.5
m) and an average particle diameter of 2.8 mm. The same measurements were made and
the data were analyzed similarly. Table 2 shows the length of unused bed for carbon
dioxide determined for each adsorbent and the % nitrous oxide removed. The % nitrous
oxide removed is defined as the % of the nitrous oxide in the feed gas which is removed
between the start of adsorption with a regenerated bed and the time when the carbon
dioxide concentration in the adsorber outlet reached 1 ppmv.
Table 2
Adsorbent |
Length of unused bed
for carbon dioxide,
inches |
% nitrous oxide
removal |
CaX |
17 (43 cm) |
97 |
13X |
13 (33 cm) |
52 |
[0052] The results show that the 13X zeolite unexpectedly has a shorter length of unused
bed and therefore a shorter mass transfer zone than the CaX zeolite, even though CaX
has a higher carbon dioxide equilibrium capacity than 13X.
EXAMPLE 2
[0053] The adsorbent bed of Example 1 was modified by placing a 12 inch layer of 13X on
top of a 3 ft (0.9 m) layer of CaX adsorbent. The above experiment was repeated wherein
the gas flow was upward, passing through the CaX and 13X adsorbent layers in that
order. The data were obtained and analyzed in the same way, and the results are shown
in Table 3.
Table 3
Adsorbent |
Length of unused bed
for carbon dioxide,
inches |
% nitrous oxide
removal |
CaX |
17 (43 cm) |
97 |
CaX followed by 13X |
13 (33 cm) |
86 |
[0054] It is seen that the length of unused bed and thus the length of the mass transfer
zone is shorter for the combination of CaX and 13X adsorbents than for the CaX adsorbent
alone. This shortening of the length of unused bed was unexpected since CaX has a
higher carbon dioxide equilibrium capacity than 13X.
[0055] Thus the use of CaX followed by 13X adsorbent of the same particle size according
to the present invention yields a shortened length of unused bed for carbon dioxide
and has several advantages over the use of a small particle size adsorbent layer on
top of a large particle size adsorbent layer operated in upflow mode as disclosed
in the prior art described above. The preferred embodiment of the present invention
reduces the total pressure drop through the adsorbent bed, reduces the potential for
adsorbent fluidization, and reduces the resulting potential for attrition and particulate
carryover.
[0056] CaX also has a higher selectivity for nitrous oxide over carbon dioxide than 13X.
This means that the use of CaX followed by 13X removes a much larger proportion of
nitrous oxide than use of 13X alone. The good performance relies on choosing adsorbents
with suitable thermodynamic and kinetic properties. In the present case, CaX provides
good equilibrium adsorption of nitrous oxide and carbon dioxide but has a long mass
transfer zone due to poor kinetic performance. The use of 13X provides a surprising
sharpening of the carbon dioxide mass transfer zone and thus improves the overall
kinetic performance in adsorption of carbon dioxide.
1. A method for removing a first and a second minor component from a gas mixture comprising
the first and second minor components and one or more major components, which method
comprises the steps of:
(a) providing a first adsorbent zone containing a first adsorbent material and a second
adsorbent zone containing a second adsorbent material, wherein the selectivity of
the first adsorbent material for the first minor component relative to the second
minor component is greater than the selectivity of the second adsorbent material for
the first minor component relative to the second minor component, and wherein the
average particle diameter of the first adsorbent material and the average particle
diameter of the second adsorbent material are substantially the same;
(b) passing the gas mixture comprising the first and second minor components and the
one or more major components through the first adsorbent zone and subsequently through
the second adsorbent zone; and
(c) withdrawing from the second adsorbent zone a purified gas containing the one or
more major components and depleted in the first and second minor components.
2. The method of Claim 1 wherein the first minor component is nitrous oxide and the second
minor component is carbon dioxide.
3. The method of Claim 1 or Claim 2 wherein the one or more major components comprise
oxygen and nitrogen.
4. The method of Claim 3 wherein the one or more major components comprise air.
5. The method of any one of the preceding claimswherein the average particle diameter
of the first adsorbent material is between about 85% and about 115% of the average
particle diameter of the second adsorbent material.
6. The method of any one of the preceding claims wherein the average particle diameter
of the first adsorbent material is between about 0.5 mm and about 5 mm.
7. The method of any one of the preceding claims wherein the first adsorbent material
comprises CaX zeolite and the second adsorbent material comprises 13X zeolite.
8. The method of any one of the preceding claims wherein the gas mixture further comprises
water and an additional adsorbent zone is provided prior to the first adsorbent zone,
and wherein the additional adsorbent zone contains adsorbent material which selectively
adsorbs water from the gas mixture prior to the first adsorbent zone.
9. The method of any one of the preceding claims wherein the feed gas is provided at
a temperature between 0°C and 50°C.
10. The method of any one of the preceding claims which further comprises terminating
steps (b) and (c) and regenerating the first and second adsorbent materials by passing
therethrough a regeneration gas at a temperature between 80°C and 400°C.
11. A system for removing a first and a second minor component from a gas mixture comprising
the first and second minor components and one or more major components, which system
comprises:
(a) an adsorber vessel having a first adsorbent zone containing a first adsorbent
material and a second adsorbent zone containing a second adsorbent material, wherein
the selectivity of the first adsorbent material for the first minor component relative
to the second minor component is greater than the selectivity of the second adsorbent
material for the first minor component relative to the second minor component, and
wherein the average particle diameter of the first adsorbent material and the average
particle diameter of the second adsorbent material are substantially the same;
(b) an inlet for passing the gas mixture into the adsorber vessel such that the gas
mixture passes through the first adsorbent zone and subsequently through the second
adsorbent zone; and
(c) an outlet for withdrawing from adsorber vessel a purified gas containing the one
or more major components and depleted in the first and second minor components.
12. The system of Claim 11 wherein the first minor component is nitrous oxide and the
second minor component is carbon dioxide.
13. The system of Claim 11 or Claim 12 wherein the one or more major components comprise
oxygen and nitrogen.
14. The system of Claim 13 wherein the one or more major components comprise air.
15. The system of Claim any one of Claims 11 to 14 wherein the average particle diameter
of the first adsorbent material is between about 85% and about 115% of the average
particle diameter of the second adsorbent material.
16. The system of any one of Claims 11 to 15 wherein the average particle diameter of
the first adsorbent material is between about 0.5 mm and about 5 mm.
17. The system of any one of Claims 11 to 16 wherein the first adsorbent material comprises
CaX zeolite and the second adsorbent material comprises 13X zeolite.
18. The system of any one of Claims 11 wherein the gas mixture further comprises water
and an additional adsorbent zone is provided prior to the first adsorbent zone, and
wherein the additional adsorbent zone contains adsorbent material which selectively
adsorbs water from the gas mixture prior to the first adsorbent zone.
19. A process for removing one or more constituents from a feed gas stream to form a purified
gas stream, comprising the steps of:
passing the feed gas stream through a bed of particles of a first adsorbent and removing
a portion of one of the constituents from the feed gas stream; and subsequently
passing the feed gas stream through a bed of particles of a second adsorbent and removing
another portion of the one of the constituents from the feed gas stream;
wherein the bed of the second adsorbent has a shorter mass transfer zone for the
one of the constituents than the bed of the first adsorbent under the same conditions,
and the particles of the first and second adsorbents have substantially the same average
diameter.
20. A process as claimed in Claim 19, wherein the particles of the first and second adsorbents
each have an average diameter of 2 to 5 mm.
21. A process as claimed in Claim 19 or Claim 20, wherein the one of the constituents
is carbon dioxide.
22. A process as claimed in Claim 21, wherein another of the one or more constituents
is nitrous oxide.
23. A process as claimed in any one of Claims 19 to 22, wherein the feed gas stream comprises
one or more gases selected from the group consisting of air, nitrogen, oxygen, carbon
monoxide, helium, methane, hydrogen and argon.
24. A process as claimed in any one of Claims 19 to 23, wherein the feed gas stream is
at a pressure of 3 to 20 atmospheres.
25. A process as claimed in any one of Claims 19 to 24, further comprising the step of
regenerating the first and second adsorbents with a regenerating gas.
26. A process as claimed in Claim 25, wherein the regenerating gas comprises one or more
gases selected from the group consisting of oxygen, nitrogen, methane, hydrogen, argon,
helium.
27. A process as claimed in any one of Claims 19 to 26, further comprising the step of
passing the feed gas stream over a desiccant before passing the feed gas stream over
the first adsorbent, wherein the desiccant is selected from the group consisting of
alumina, silica gel, impregnated aluminas, A zeolites and X zeolites.
28. A process as claimed in any one of Claims 19 to 27, further comprising the step of
cryogenic distillation of the purified gas stream.
29. A process for removing a first and a second constitutent from a feed gas stream to
form a purified gas stream, comprising the steps of:
passing the feed gas stream through a bed of particles of a first adsorbent, the first
adsorbent having a selectivity for the first constituent over the second constituent
of 0.45 or greater; and subsequently passing the feed gas stream through a bed of
particles of a second adsorbent;
wherein the bed of the second adsorbent has a shorter mass transfer zone than
the bed of the first adsorbent for the second constituent under the same conditions,
and the particles of the first and second adsorbents have substantially the same average
diameter.
30. A process as claimed in Claim 29, wherein the selectivity of the second adsorbent
for the first constituent over the second constituent is 0.40 or less.